
t (  u)+x (  u)=I(  u)+d  u,   in    (0,)RdSd-1 ,

where, using an extension by zero, we interpret \(\phi _{\epsilon }u\)  to be defined in the whole space. As a consequence, \(\phi _{\epsilon }u\)  satisfies the RTE () with a source \(F:=\theta _{d}\,\phi ^{\prime }_{\epsilon }\,u\)  for which we can apply the argument presented in [1]}.
